OpenSIPS is a mature implementation of a SIP
server/proxy. It is more than a SIP proxy/router,
as it includes application-level functionalities.
OpenSIPS, as a SIP server, can server as the core
component of any SIP-based VoIP solution.

The GNU Gatekeeper is a free H.323 gatekeeper
based on the OpenH323 project. You can use it to
manage a Voice-over-IP network and let endpoints
(e.g., Netmeeting) communicate through symbolic
names. It also has an external interface for
billing and other applications. It runs on a
number of Unix versions (including Linux and
Solaris) and Windows.

Yate is a next-generation telephony engine. While currently focused on Voice over Internet Protocol (VoIP) and PSTN, its power lies in its ability to be easily extended. It supports SIP, H.323, IAX, MGCP, Jingle, Jabber, E1, T1, analogic, robbed bit, ISDN PRI, BRI, and SS7. YateClient is an Instant Messenger and Voice application for Jabber/Jingle IM, SIP, H.323, and IAX VoIP protocols.

QueueMetrics is a queue log analyzer that measures
a number of metrics of queue and agent activity
for the Asterisk PBX. It offers detailed reports,
real-time panels, and start pages for agents. It
also drives a wallboard and has an XML-RPC data
interface.

WombatDialer is a platform to provide mass outbound calling. This can be used to implement many different services. By offering a set of ready-to-use components and a monitoring GUI, it lets you create complex solution in minutes. It can work on predefined call lists or can dynamically create them over an API (e.g., dial number X after 10:30AM). It shares the load on one or more PBX servers and has flexible rescheduling logic to handle missed calls. It is built to be used with your existing Asterisk PBX, and does not require separate servers or a separate set of lines. It can call over VoIP or through the public telephone network. It is built to integrate with your business processes, and can receive calls to be made over HTTP and/or notify an external system in realtime of calls made and results gathered. It works natively with the QueueMetrics Call-Center Monitoring Suite to produce state-of-the-art campaign analyses and insight.

WinTariff is a program that collects and processes information about telephone calls. It receives data from an office PBX about the date/time, duration, and dialled number of phone calls, then calculates the cost of the call and determines the direction. Various reports about all calls during the month, calls to a certain internal subscriber, etc. can be generated. A data collection module is available for Linux/FreeBSD.